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O.K. I'm talked out of panning supply duct. How about leaving a floor joist in the air flow of the supply from air handler through the floor? What does code say? What if there is no code to satisfy, is it effecient, safe? What if it is covered with sheet metal? One 2x8 joist.

contact your local code enforcement office, i.e. building inspector, hvac, etc. They will be able to tell you exactly what applies in your area. Many of us on this site are scattered all across the U.S. and what applies in our area may not apply in yours. What you have been "talked out of" is a code-violation! Are you installing this duct system yourself?

I'm starting to invision by the questions of using cavity space for supply air and can a metal covered joist be in there that the outcome for the total system will be nothing short of a mechanical design disaster.

Ever see the face of a person that is demonstrated to that some or all of their entire install needs torn out,redesigned,reinstalled to work properly, stop break downs or to be safe in their home?

"The Bitterness of poor quality remains long after the sweetness of low price is forgotten"

Codes

This is in a rural area and there are no codes to satisfy and no inspectors, it is up to the HVAC contractor to do a good job. The contractors I've called walk in, without doing any calculations except ask for total square footage and make their size determination. They all seem to have one standard size of duct work that they put together and I haven't found any professionalism around here yet. Then they want to charge an exorbant price. This house already has most of the duct work and I'm relocating a new airhandler to the center of the house. No I'm not going to do it without a professional advising and helping on a part time basis. I've already determined by research that two bedrooms don't have enough supply air. The return will be at the air handler with no duct run. Are my questions too dumb to participate on this forum? I've already reworked the dryer vent which went into the wall cavity and came out under the house before going to 4" round to outside. There was dryer lint in the wall cavity it and trying to blow out around an electrical outlet 4' high on the wall. They are lucky the house didn't burn.

needed will probably be joist hangers, dbl headers-- hard to say without a drawing -- one has to carry the load somehow from the missing joist -- it would be safest to sister the side joists also --
1 or 2 stories?
how long is joist?
what is the wood?
what is its selection?
where will the cut be with respect to one end?
what additional load will it have to carry in addition to what was there?
how is the old joist fastened at each end?
are side joists in good shape?
what is the consequence of failure?

Maybe the professional you've talk with have years of experience and feel comfortable with their proposal. If so, have them put it in writing that the system with work as designed. The path your following is leading you towards a system full of problems for years to come, code or no code.

the dumb question is the one?

That is un asked, and the dumber one is the one who conemns you for asking it! they cant relate or lact the intellect to understand, how they can help resolve the prob, and usually are the prob! dont ever feel that way!